This invention relates to a vector signal generator capable of providing, for example synthesized signal having a frequency range of 10 MHz to 3 GHz, with wideband modulation capabilities, including calibrated scalar, digital, burst, and vector modulation capabilities not available in the prior art signal generators.
Over the last decade, there has been a steady growth in the complexity of modulation employed in Radio Frequency (RF) and microwave systems. Some of the driving forces behind this growth are advances in integrated circuit (IC) technology, the need for more efficient use of bandwidth, availability of wideband and high frequency devices, and the quest for superior system performance in electronic warfare (EW) and radar technologies. Accompanying this growth is the need to generate or simulate these complex modulation signals. Traditional signal generators, with their simple AM and FM modulation capabilities and limited modulation bandwidths, do not meet this need. Up to now the approach taken by test engineers when faced with the task of generating complex modulation is to build their own equipment which is very dedicated, works only over a small range of carrier frequencies and/or data rates, and is difficult to document and maintain, as well as being expensive.
"Vector modulation", sometimes called IQ modulation, refers to the independent modulation of the I (in phase) and Q (quadrature phase) components of a carrier signal. Vector modulation is a very powerful technique because it may be used to generate modulations of any arbitrary phase and magnitude modulation with respect to a reference carrier.
"Scalar modulation" refers to modulation of the carrier signal envelope or amplitude.
"Digital modulation" is a term used in terrestrial and satellite communications to refer to modulation in which digital states are represented by phase and amplitude states of the carrier. Digital modulation is a special case of vector modulation because each modulation state can be decomposed into I and Q components.
"Burst modulation" of a carrier is similar to pulse modulation and may be used to gate the carrier on and off using a burst modulation digital input.
The I or Q "accuracy" is the actual I or Q modulation level produced by the generator compared to the theoretical levels, as shown in FIG. 1.
The I or Q "frequency response" is the flatness of the I or Q modulation level. Since I and Q are measured individually in this measurement, a power meter is a good test instrument.
"Crosstalk" is the modulation that occurs on I when only Q is modulated or vice versa. Crosstalk at modulation rates close to DC are measured as part of the I and Q accuracies as depicted in FIG. 1.
A conceptual block diagram of a typical vector modulator 10 is shown in FIG. 2. A carrier signal of desired frequency is received on input terminal 11, typically from a frequency synthesizer referenced to an oven-stabilized timebase (not shown). When the carrier signal first enters vector modulator 10, it is split into I and Q components, for example, by a 90.degree. hybrid phase shifter 12 well known in the art. The level and phase polarity of each carrier component I and Q is controlled in mixers 13 and 14 by analog baseband signals. The modulated I and Q signals from mixers 13 and 14 are then combined in summing device 15 which is, for example, a summer or combiner well known in the art.
However, while such prior art vector modulators are suitable for use at fixed operating frequencies where harmonics can be filtered from the output signal using fixed frequency filters, such vector modulators are not well suited for use over a wide frequency range.